The field of the present invention relates to diffraction gratings. In particular, monolithic surface diffraction gratings are disclosed herein that include integrated calibration features.
Diffraction gratings have long been used to disperse light into its spectral components for scientific analysis or for other reasons. Traditional surface gratings (reflective or transmissive) comprise closely spaced sets of grating lines or diffractive elements that are typically created either by a mechanical ruling engine or by recording an optical interference pattern via exposure of a photosensitive material. The mechanical and interferometric patterning methods have drawbacks in that only limited variation in the spacing, curvature, and other properties of the diffractive contours is practically achievable. Recently, the resolution of Deep Ultraviolet (DUV) photolithographic patterning tools has reached and exceeded the values needed to make diffraction gratings useful in the visible and near-infrared portions of the electromagnetic spectrum. Owing to the nature of the photolithographic patterning process, practical constraints are eliminated or relaxed for diffractive element geometry (e.g., structural parameters such as cross-sectional shape, contour shape, contour spacing, and so on) and other properties. The diffractions gratings disclosed herein employ design considerations that are readily implemented with photolithographic patterning and that include creating multiple diffraction gratings on single grating substrates. The multiple diffraction gratings thus formed can provide convenient calibration of output spectra and simultaneous viewing of adjacent spectral regions so as to span wide bandwidths at high resolution.
Various embodiments, implementations, and adaptations of diffractive elements of waveguide gratings or surface gratings (reflective or transmissive) are disclosed in:                application Ser. No. 11/685,212 filed Mar. 13, 2007 (now U.S. Pat. No. 7,286,732);        application Ser. No. 11/532,532 filed Sep. 17, 2006 (now U.S. Pat. No. 7,190,859);        application Ser. No. 11/531,274 filed Sep. 12, 2006 (now U.S. Pat. No. 7,519,248);        application Ser. No. 11/423,856 filed Jun. 13, 2006 (now U.S. Pat. No. 7,224,867);        application Ser. No. 11/376,714 filed Mar. 14, 2006 (now U.S. Pat. No. 7,349,599);        application Ser. No. 11/298,290 filed Dec. 9, 2005 (now U.S. Pat. No. 7,330,614);        application Ser. No. 11/280,876 filed Nov. 15, 2005 (now U.S. Pat. No. 7,773,842);        application Ser. No. 11/076,251 filed Mar. 8, 2005 (now U.S. Pat. No. 7,062,128);        application Ser. No. 11/055,559 filed Feb. 9, 2005 (now U.S. Pat. No. 7,123,794);        application Ser. No. 10/998,185 filed Nov. 26, 2004 (now U.S. Pat. No. 6,993,223);        application Ser. No. 10/989,236 filed Nov. 15, 2004 (now U.S. Pat. No. 6,965,716);        application Ser. No. 10/653,876 filed Sep. 2, 2003 (now U.S. Pat. No. 6,829,417);        application Ser. No. 10/602,327 filed Jun. 23, 2003 (now U.S. Pat. No. 6,859,318);        application Ser. No. 10/229,444 filed Aug. 27, 2002 (now U.S. Pat. No. 6,678,429);        application Ser. No. 09/811,081 filed Mar. 16, 2001 (now U.S. Pat. No. 6,879,441).        
Application Ser. No. 11/531,274 and application Ser. No. 11/376,714 are hereby incorporated by reference as if fully set forth herein. Those applications and the other applications listed are indicative of the state of the art available for forming diffractive elements, and may be applicable to formation of diffractive elements for monolithic arrays of diffraction gratings according to the present disclosure.
A simple example of a diffraction grating 100 is shown in FIG. 1A. The example grating comprises a flat surface (i.e., a grating substrate, assumed to lie in the xy-plane) having a complex surface reflectivity (i.e., reflectivity characterized by amplitude and phase) that varies spatially, typically periodically. A reflection grating is considered in this example, but the general treatment applies to the complex transmissivity of a transmission grating as well. The surface-normal vector is n (assumed parallel to the z-axis). A flat grating is considered in this example, but more generally the grating surface can be curved and then the surface-normal vector would be position-dependent and defined relative to a plane locally tangent to the grating surface. The grating complex reflectivity can vary in amplitude, phase, or both as a function of position on the grating surface. As shown in the simple grating schematically depicted in FIG. 1A, reflectivity varies only along the x-axis. The spatial variation of reflectivity arises from diffractive elements 101 formed on or within the grating substrate. Such diffractive elements can take myriad forms. Typical examples include, but are not limited to: grooves or ribs formed on the grating surface, in or on a material layer on the grating substrate, or at a material interface within the grating substrate; or refractive index variations in the grating substrate material or in a material layer deposited thereon.
A region of constant reflectivity is defined as a diffractive contour, and the diffractive elements can be said to follow such contours or to be defined with respect to such contours. In the example grating of FIG. 1A the diffractive contours are simply straight lines parallel to the y-axis. The orientation of the grating surface and diffractive contours in FIG. 1A are chosen for expositional convenience only. Straight diffractive contours are considered in this example, but more generally can follow curvilinear paths. The corresponding diffractive elements can be continuous (as in the example of FIG. 1A) or they can be segmented or otherwise only partially fill the corresponding contour to enable control of effective diffractive element reflectivity, provide for the overlay of multiple grating structures, or for other reasons (as described in some of the references listed or incorporated above). Similarly, the positions of small subsets of diffractive elements can be configured so as to control the net diffracted field from each subset (as described in some of the references listed or incorporated above).
The grating possesses a wavevector Kg which lies in the grating plane (i.e., the xy-plane in this example) and is oriented normal to the diffractive contours. The magnitude of Kg is 1/a, where a is the spacing between contours measured along a mutual normal direction. For the example grating of FIG. 1A having uniformly spaced straight diffractive contours, the grating wavevector Kg is constant. More generally, e.g., for gratings having curved or variably spaced contours, the grating wavevector Kg can be defined only locally over small regions where contour spacing and orientation are relatively constant. Alternatively, a grating may be described in terms of a spatial Fourier transform which provides a decomposition of the structure in terms of multiple wavevectors.
Monochromatic input light (wavelength λ) incident on the grating possesses a wavevector kin oriented normal to its wavefront, i.e., kin is parallel to the ray representing the input light. In cases where the input light has a spatially varying wavefront, its wavevectors can be defined locally. The wavevector kin has the magnitude 1/λ. When the input light has a range of spectral components, wavevectors of a corresponding range of magnitudes represent the various spectral components.
In the simple case where Kg, kin, and n lie in a common plane (i.e., when Kg lies in the plane of incidence defined by kin and n), the diffraction geometry illustrated in FIG. 1B results wherein the grating properties (the spacing a), input and output directions (θin and θout), and wavelength (λ) are related according to the equation:mλ=a sin θin−a sin θout  Eq. 1where m is any integer including zero that provides real solutions for the output angle (defined as positive when on the opposite side of the normal relative to the input angle). Since Eq. 1 includes the wavelength λ of the incident light for m≠0, the output angle will vary with input wavelength (i.e., the grating exhibits angular wavelength dispersion).
In more general cases, including those case wherein the grating wavevector does not lie in the plane of incidence defined by kin and n, the output wavevector may be determined by decomposing the input wavevector into two parts, one parallel to the plane of the grating and one perpendicular to it. These components are denoted {right arrow over (k)}inp and kinz, respectively. Analogous components for the output wavevector are {right arrow over (k)}outp and koutz. The allowed values of these quantities are given by:{right arrow over (k)}outp={right arrow over (k)}inp+m{right arrow over (K)}g  Eq. 2akoutz=√{square root over ((kin2−koutp2))}  Eq. 2bwhere m is any integer (including zero) that results in a real value for of kZout. Eqs. 2a and 2b indicate that a single input beam generates one or more output beams and except for the beam corresponding to m=0, the output directions are wavelength dependent. The m=0 beam is the specular reflection expected if the grating were a smooth surface without diffractive elements. The number of output beams is determined by the magnitude and orientation of Kg relative to kpin.
An exemplary implementation of a reflective diffraction grating is shown in FIG. 2A, which shows the geometry of the grating 200, input beam 202, and various output beams 204. The grating surface defines the xy-plane, while the plane of incidence defines the xz-plane. The output beams 204 include the specular reflection (m=0) having a direction in the xz-plane determined by the standard law of reflection. Non-zero diffracted output orders (i.e., beams for which m≠0) are also found in the xz-plane, where light of various wavelengths is directed in directions consistent with Eq. 1.
When viewed in the far-field, the grating output typically assumes the general form shown in FIG. 2B when the input includes a broad spectrum of wavelengths. There is a central spot 210 representing the specular reflection and stripes 212 representing dispersed non-zero diffractive orders (m=±1) on either side of the specular reflection. Generally, shorter wavelength components of the output beams are closer to the specular reflection. Additional higher orders (|m|>1) may also appear depending on the wavelength range of interest and the spacing between grating lines.
In the arrangement of FIGS. 2A and 2B, a reference quasi-monochromatic light source (e.g., a laser based on atomic transitions, a spectral lamp, or other source of known narrow spectral output) incident on the grating will produce distinct spots in the bands of diffracted light 212. Such single output beams provide a single absolute calibration reference for the dispersed, diffracted output. At least two absolute calibration references would be required to determine absolute placement and absolute spatial dispersion of the diffracted, dispersed output. This can be accomplished by a reference optical signal having two or more quasi-monochromatic wavelength components, for example.